Development of mass spectrometric methods for proteomics analysis utilizing gas-phase chemistry and ultraviolet photodissociation
The utility of ultraviolet photodissociation (UVPD) in comparison to higher-energy collisional dissociation (HCD) to provide sequence coverage was assessed for various protein cation charge states and sizes. UVPD provided consistently higher sequence coverages through more uniform fragment ion distribution along the protein sequence. HCD provided lower sequence coverage values as well as more preference towards cleavage at the most labile bonds. Assessment of coverage dependence at lower charge states was also performed through proton transfer reactions (PTR) with ion parking. Overall, HCD provided preferential cleavage C-terminal to amino acids with acidic sidechains and N-terminal to proline, while UVPD provided more evenly distributed cleavage sites with enhancement near proline and phenylalanine. Using UVPD as a structural analysis tool, PTR was assessed for perturbations to native-like structure of various protein complexes. Through comparison of UVPD fragment intensities, spectra of protein complexes generated through PTR showed little difference to spectra obtained from native-like protein spectra. Following this, PTR-UVPD was applied to elucidate fragment origins of an ambiguous homodimeric protein complex, otherwise displaying complex a complex mass spectrum of overlapping species. A novel approach to performing UVPD using light emitting diodes (LEDs) was explored involving the engineering of a new planar ion trap. Commercially available ultraviolet LEDs emitting photons with wavelengths ranging from 255 to 275 nm were obtained and interfaced with the new ion trap. Sequestration of sample ions in a small spot allowed optimization of overlap with LED photons and resulting fragmentation efficiencies were assessed. Once optimized, LED-UVPD was successfully performed for electron photodetachment (EPD) of single stand DNA and tyrosine sidechain cleavage of a peptide. Custom instrument function was enabled to automatically resonantly eject un-dissociated precursor ions following UVPD and was applied during liquid chromatography (LC) bottom-up proteomics experiments. Through ejection of uninformative, un-dissociated precursor ions, detrimental mass shifting effects caused by increasing the number of charges during spectrum acquisition were relieved. It was observed that performing PE-UVPD resulted in higher protein identification confidence values than for UVPD alone for an E. coli whole cell lysate digest.